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The Ericsson XY Switch
In 1938 the L.M. Ericsson Company of Sweden developed a new exchange switch, called the XY Switch. It has 100 points much like a Strowger type switch. It shares some common features with its bigger brother, the Ericsson 500-point switch.
Its smaller capacity is graduated in decimals (10x10) and is powered by electromagnets instead of motors. The reason for the XY moniker is because the contact wipers move in two separate directions at right angles to each other, along the X and Y coordinates.
It was intended for use in smaller exchanges where the 500-line selector version was overkill due to its larger capacity or in places where the overhead of motor drive mechanics was not warranted.
Fig 1, XY Switch mechanism frame circa 1937 [Ericsson]
About 1945, Ericsson signed a license agreement with the American telecom company Stromberg-Carlson, which used the XY Switch for rural telephone stations. The first US exchange using the XY Switch went into operation in 1947. For this reason, this switch is also called a Stromberg-Carlson XY switch. The switch in this section is referred to as an Ericsson XY despite some pictures being of the Stromberg-Carlson version.
Under the hood
Interestingly, this switch “feels like a Strowger.” In what ways?
• 10x10 numbering --- 100 terminal positions per bank (200 terminals, double sided wipers)
• Has two degrees of motion
• Directly responds to 2 dialed digits or hunts, if required
• No need for auxiliary logic to separately record the digits and control the switch
In fact, an exchange could have a mix of Strowger type and XY switches and they would seamlessly work together. This is one reason why the term “step-by-step” describes a switch design style and applies to more than the Strowger type. More on this below.
This switch is categorized on the switch page under “rotary/linear.” The mechanism internals have linear and rotary movements, but the wiper arms have only linear movement.
Fig 2, Breakdown of XY Switch internals [Ericsson]
Referring to Fig 2, the X+Y movements are obtained with two ratchets: one for the X movement and one for the Y movement. The ratchets are activated by means of stepping magnets, XM and YM. Magnet ZM restores the wipers to home under spring power.
Fig 3, XY Switch with 3 motion control electromagnets [JKL]
The photo in Fig 3 clearly shows the three electromagnets. Notice the “rack and pinion” that moves the wipers in the Y direction due to the rotary motion of the “toothed sleeve” actuated by YM.
The video below shows the mechanism's carriage moving in X and Y directions. There are several demos in sequence. The first one shows manually pressing the ZM (release) magnet then pressing the XM magnet eight times. The other demos show wiper motions making contact with the detached terminal banks.
Ericsson XY in motion. Sources: JKL and Phil McCarter
Switch anatomy
As mentioned, the XY switch can replace any Strowger-type in a step-by-step exchange. The switches are interoperable in a sense.
Here is a high-level structural comparison:
• Strowger-type switch-- Integrated: control relays and mechanism and wipers, with separate: terminal banks
• Ericsson XY switch-- Integrated: mechanism and wipers, with separate: control relays and terminal banks.
So, the XY mechanism is identical when used as a line finder, selector, or a final connector switch. For a Strowger-based exchange, there are four different switch types because the relays are integrated into the switch frame.
In the XY system, relays are stacked in a rack near the mechanism. The relay logic differentiates a switch’s functions. Fig 4 shows eight relays associated with a single XY Selector type switch.
Fig 4. Eight relays supporting one XY mechanism [JKL]
Fig 5 Shows a rack of XY mechanisms (pancake style) each with a 30-pin connector to interface to its respective relays.
Fig 5. Eight XY switch mechanisms in an open cabinet [JKL]
The overall packing density is slightly better for an XY system compared to Strowger-type. It's a plus that there is only one XY switch mechanism type needing maintenance and replacement.
Fig 6 shows ten relay shelves (8 relays per shelf) on the left and XY switch mechanisms, stacked in a cabinet, on the right. This collection is equivalent to 10 Strowger-type switches. Notice a large removable rectangular plug associated with each XY switch.
Fig 6. XY switch mechanism cabinet (right) with associated relays (left) [JKL]
The XY switch was eventually phased out in the early 1980s as electronic switching systems became more widely used. However, many XY switches remained in service until the early 2000s.
References
Ericsson Review, #1, 1938. www.ericsson.com/en/about-us/history/sources/lme-review
General Dynamics (Stromberg Carlson was a division), Fundamentals of XY Dial Systems, 1956
JKL Museum of Telephony, Santa Cruz, Ca. Thanks to Remco Enthoven for advice, providing some photos, and assistance with video capture.
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